<!DOCTYPE html>
<html lang="en">
 
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .border {
            width: 300px;
            height: 300px;
            border-radius: 20px;
            margin: auto;
            box-sizing: border-box;
            border: 1px solid transparent;
            background-image: linear-gradient(#fff, #fff),
                linear-gradient(to bottom left,
                    rgba(255, 216, 64, 1),
                    rgba(243, 172, 255, 1),
                    rgba(138, 236, 255, 1));
            background-origin: border-box;
            background-clip: content-box, border-box;
            border-radius: 20px;
        }
    </style>
</head>
 
<body>
    <div class="border"></div>
</body>
 
</html>